Do new blocks appear in old world?

I've seen that new features appear in worlds that I created previously (like falling sand), but will I find new blocks when I mine underground, or were all the blocks defined when I first created the world? If not, then when do they get created? Is it only when I get to within a certain distance of them?

I've seen that new features appear in worlds that I created previously (like falling sand), but will I find new blocks when I mine underground, or were all the blocks defined when I first created the world? If not, then when do they get created? Is it only when I get to within a certain distance of them?

In other words, to get new blocks, do I have to create a new world?

When you first create a new world the chunks are generated and saved. Nothing new appears after that, you have to make a new world if you want new generation features and such.